Switching circuit



Dec- 31, 1940- J. o. SMETHURST 2,226,952

SWITCHING CIRCUIT F'iled Dec. 13, 1959 2 Sheets-Sheet 1 VVE/WOR J. O. SME THURS 7' 5v A T rom/EP J. o. sMETHuRsT 2,226,952

SWITCHING CIRCUIT .Filed Dec. 13, 1939 2 Sheets-Sheet 2 Dee. 31, 1940.

/Nl/E/v J. 0. SME THUR ATTORNEY AAA Patented Dec. 31, 1940 PATENT OFFICE 2,226,952 swlroHINo omoorr Joseph 0.'Smethurst, Bloomfield, N. J., assgnor `to Bell Telephone Laboratories, Incorporated, New York, N. Y.,'a corporation of New York Application December 1s, 1939, serial Nc. 309,029

4 Claims.

This invention relates to switching circuits :and particularly to 'signal-controlled switching circuits employing electromagnetic relays.

The invention is more specifically directed to, i5 although not limited to, signal-controlled switching circuits employed in connection with the oppositely-directed.repeating paths of a.twoway signal 'transmission system, such as aradio or wire telephone system, for suppressing echoes and preventing singing. Such switching circuits usually employ an amplier-detector for amplifying and detecting the applied signals and electromagnetic relays or other switching devices operated by the detected signals to insert or remove losses in the transmission circuits to provide the desired control of transmission.

An object of the invention is to improve such switching circuits. i c

This object is attained in accordance with the invention by relatively arranging and controlling the -transmission apparatus in the isignal-controlled switching circuits so as to'provide more efcient operation of these circuits with economy of apparatus.

of the windings of one or more switching relays of a signal-controlled switching circuit, in a balanced bridge circuit arrangement and to control the balance of said bridge in such manner as to provide faster operation of the switching relays with desired control of their ,hang-overcharacteristics, with a small number of relays.

Other objects and features of the invention will be understood from the following detailed drawings, Figs. 1 and 2 of which show schematically portions of two-way telephone systems` of different types employing switching `circuits embodying the invention.

40 In Fig. 1 the switching circuits of the invention are shown associated with a control termi-V nal for a two-way radio telephone system, only those portions of the terminal apparatus and circuit required to explain the inventionbeing illustrated.

The control terminal of Fig. 1 as .shown in-` and the output of the circuit RC being coupled.

in well-known manner by a hybrid coil H1 and associated balancing network N1 toa' two-wire 65 circuit which may be connected by switchboard A feature of the inventionis the connection.

description when read in conjunction with kthe.

SW with the two-way telephone line TL. As shown, the transmitting circuit TC includes in itsinput a volume control device VC1, which may be afvogad (volume-operated gain-adjusting device) of any of the well-known types, the main 5 function of which is to control the transmitted signal volume level to prevent overloading the radio transmitter. The receiving circuit RC includes in its, output a similar volume control device VCz. 10

In the absence of outgoing signal transmission the transmitting circuit TC is disabled or blocked at the point l by a Vshort-circuiting connection across that circuit, and the receiving circuit RC is normally operative due to the normal break in 15 A the short-circuiting connection across that circuit at the point 2,

The short-circuiting connection across the transmitting circuit TC' at the point l is opened to unblock that circuit for outgoing signals, and 20 the normally open short-circuiting connection acrossthe receiving circuit RC at the point 2 is closed to block or disable that circuit so as to suppress echoes and prevent singing, under oontrol of the transmitting portion TV of a vodas 25 (voice-operated device anti-singing) switching circuit, the input of vwhich is connected across the circuit TC at a point in the output of the volume control device V01 by the hybrid coil Hs and associated balancing network N2. The transmitting vodas TV includes a sensitivity-adjusting loss pad 3 in its input, the ampliiier-detector 4 and a chain of relays including the master relays TM1 and TMz and switching relays S1, S2 and S3, controlled by operation of the amplifier-detector 4.

False operation of the transmitting vodas TV when incoming signals are being received at the terminal is prevented by the receiving portion RV of the vodas, the input of which is connected across the receiving circuit RC at a point 5 to the left of the switching point 2. The receiving vodas RV includes the sensitivity-adjusting loss pad 6 in its input, the ampliiier-detector 1 and v a chain of relays, including master relay RM and the switching relays S4 and S5, controlled from f the output of amplifier-detector 1, operating to disable the transmitting vodas TV andthe volume control device V01 in the transmitting circuit TC 50 in the manner to be described.

The detector portion of the ampliiier-detector 4 in the transmitting vodas TV comprises a single electron discharge tube 8 of the pentode type, comprising a heated cathode 9, the anode l0, a

S2 and S3, respectively,

control grid I I, a screen grid I2, and a suppressor grid I 3 which is connected directly to the cathode. The control grid-cathode circuit of tube 8 is coupled to the output of the amplier portion of the ampliiier-detector 4 through resistance I4, and the tube 8 is normally biased beyond cut-off by a negative potential applied to the control grid II from a voltage divider connected across the grid battery I5'. Plate current is supplied to the anode I Il of detector tube 8 from plate battery I6 through the normally closed left-hand contacts of the receiving vodas master relay RM, resistance I'I and the operating windings 26 and 25 of the transmitting master relays 'IMz and TMi in series.

The detector portion of the amplifier-detector 'I in the receiving vodas RV comprises a single four-electrode electron discharge device. I8 comprising a heated cathode I9, an anode 2l),v a contrcl grid 2I, and a suppressor grid 22 connected directly to the cathode I 9. The control gridcathode-circuit of thel detector tube I8'is coupled to the output of they amplier portion of amplifier-detector 8in any suitable manner, and the tube I8 is normally biased beyond cut-off by a nega-tive potential applied to the control grid 2I by the voltage divider across grid battery 24, so as to operate quicklyin response to applied signals. Plate current is supplied to the anode 28 of detector tube I8 from plate battery I6 through resistance 23 and the operating winding 42 of the relay RM in series. The screen grid I2 of detector tube 8` in the transmitting amplifierdetector 4 is normally positively biased by plate battery I S through series resistance 23 so that the detector tube 8 will be quickly operated in response to signal energy applied to the input of amplier-detector 4 to operatively energize the operating windings 25 and 26. of master relays TM1 and TMz, respectively.

In the absence of signal transmission in the transmitting circuit TC, the amplier-detector 4 in the transmitting vodas TV is unoperated so that the operating windings 25 and 26 of the transmitting master relays TM1 and TMZ, respectively, are unenergiaed. Then, the biasing current applied from battery. 2'I to the biasing windings 28 and 29, respectively, of these relays in series hold the relay armatures on the right-hand contacts. The hold-over windings 3D and' 3l. of the relays TM1 and TMz are yshorted through the resistance 32, condenser 33 and resistance 34 in series. For this condition of the master relays TMi and TMs, the operating windings 35, 36 and 3'I of the transmitting switching relays Si, connected in series to the open left-hand contact of master relay TMi, are unenergized, and the biasing current from battery 38 flowing through the biasing Winding 39 of relay S1, the biasing winding 48 of relay S2, and the biasing winding 4I of relay S3 in series holds the armatures of these relays respectively on the left-hand, right-hand and left-hand contacts of the relays. The transmitting circuit TC in then blocked at the point I by the short-circuiting connection through the closed armature and right-hand Contact of relay S2, and the receiving circuit RC is in the operative condition due to the break in the short-circuiting connection across that circuit at the point 2, provided by the open right-hand contact of relay S3.

In the absence of signaly transmission over the receiving circuit RC, the amplifier-detector I of the receiving vodas RV is unoperated, and the operating winding 42 of the receiving master relay RM, connected in the plate circuit of detector tube I8, is unenergized. The biasing current from battery I6 transmitted through resistance 43 and the biasing Winding 44 of the relay RM causes the armature of that relay to be held on its left-hand contacts.

The switching relay S4 has an operating winding 45, a biasing winding 46 and a hold winding 41, andthe switching relay S5 has an operating winding 48', a biasing winding 49 and a hold Winding 50. The corresponding windings of the two relays S4 and S5 are connected in series so that the relays operate and release in unison. The operating and biasing windings of the relays S4 and Ss, the battery I6 and resistances 5I, 52, 53 and 54 are connected in a Wheatstone bridge circuit.. In this bridge circuit, the armature and closed left-hand contact of relay RM, resistance 5I and biasing winding 46 of relay Si and biasing windingl 49of relay S5 in series are connected in one arm, the resistance 52 in a second arm, and the equalreslstances 54 and 53 are respectively connected inthe other two arms of the bridge. Plate battery` I6 is connected across one diagonal of the bridge-circuit so that in the unoperated condition of the Amaster relay RM biasing current is supplied from that battery to the biasing Winding`46 of relay S4 and biasing Winding 49 of relay S5 in series. The'operating winding 45 of relay S4 and the operating winding 48 of relay S5 in series are connected across the other diagonal of the bridge circuit so that in the normal balanced condition of the bridge little current hows through the operating. windings of relays S4 and S5 and the relay armatures are respectively held on their left-hand contacts due to the biasing eiect of the biasing windings of the relays.

The purpose of the other circuit elements shown in Fig. 1 but not previously referred to will be brought out in the following complete description of the operation` of the system.

Now let it be assumedthat telephone signals received over the telephone line TL from a west subscriber are impressed by the hybrid coil Hi on the transmitting circuit TC and are transmitted over that circuit towards the disabling point I, andat that time no east-to-west telephone signals from an east telephone subscriber' associated with the east end of the system have reached a corresponding point in the receiving circuit RC.

A portion ofthe outgoing telephone signals` in the transmitting circuit TC will be diverted through hybrid coil H2 into the input of the transmitting vodas TV. These signals, with a level determined by the initial adjustment of the sensitivity-adjusting loss pad 3, will pass into the amplifier-detector 4 and will be amplified by its amplifierportion. The amplified signals will be impressed on the control grid-cathode circuit of the detector tube 8 across the resistance'l4. Because of the normal positive bias on the screen grid I2 of the tube, detector tube 8 will quickly operate to supply energizing current to the operating winding 25 of the relay TM1 and the operating winding 26 of relay TMg in series, cau."- ing the operation of these relays to shift their armatures from the right-hand to the left-hand contacts.

Relay- TM1 operated to its left-hand contact short-circuits the hold-over condenser 51, which is normallyl charged" from battery I 6, to discharge it through series resistance 58, and connects the operating windings 35, 36, and 3l' of the switching relays Si, S2 and S3 in series through the armaturesA yand left-hand contacts' of relays1TM1 and S4 across plate battery I6, causing the relays Si, S2 and S5 to operate.

TheA purpose of the relay TM2, condenser `33*- and associated resistances is to insure that any speech currents great enough to cause operation of the 'relay TM1 to break its right-hand contact f will cause it to operate for a long enough time for the hang-over condenser 51 to discharge completely. On release of the master relay TM1, the relays of the transmitting chain will then be certain to hold operated long enough to allow the weak parts of speech to pass and to preventr trouble being caused by echoes that are received from the radio channels. When relay TM2 operates to break its right-hand contact, ground is removed from condenser 33, and that condenser will then be charged up to the potential of battery 56v through the resistance 34 land winding 3l of relay TM2. This kick-up circuit forces the relay TM2 to operate instantaneously and hold against the left-hand contact for the dur-ation of the charging current. The operation of relay 'IMz also removes the ground from the junction point of resistances and 32 which allows current to iiow from battery 56 through resistances 65 and 132 to winding 30 of relay TM1 to ground, causing a positive operation of TM1. For every break of the right-hand contact and armature of TMz a positive operation of TM1 is insured to completely discharge the lhang-over condenser 51. When energizing current is removed from the operating windings 25 and 26 the master relaysv lTM1 and 'IMz will release immediately. However, the switching relays S1, S2 and S3 will not release but will be maintained operated for the interval required to charge the condenser 51 through resistances 68 and 58 and relay windings 35, 36 and 31, The final release of relay S1 will apply a ground through the contacts of S1 and TM1 to the junction of resistances 58 and 68, which discharges the condenser 51 to end the hang-over period and restore S1, S2 and S3 to their normal positions.

The operation of relay S1 to break its left-hand contact will disconnect ground from the righthand contact of master relay TM1. The operation of relay S2 to break its right-hand' contact will open the normal short-circuiting connection across the transmitting circuit TC allowing the outgoing signals to be transmitted out over the circuit TC to the radio transmitter, and the simultaneousv operation of the relay S3 to shift its armature from the left to the right-hand contact will connect a short-circuiting connection across the receiving circuit RC at the point 2, disabling that circuit so as to suppress echoes and prevent singing. When the master relay TM1 breaks its lefthand contact due to cessation of the transmitted signals, the relays S1, S2 and S3 will remain operated for a time, being held up by the charging current to the hold-over condenser 51 owing from battery I6 through the operating windings of these relays. As this condenser becomes charged, this current decreasing until, its magnetic effect on a particular relay core is exceeded by that of a biasing winding, the relay releases.

normal condition with the outgoing transmission blocked and the receiving circuit enabled' as shown in Fig. 1.V

The operation of the circuit of Fig. 1 for the case where incoming signals are received in the receiving circuit RC in the absence of prior outgoing signal transmission in the transmitting circuit TC is as follows.

`The receiving circuit RC being in its normal operative. condition at the point 2, the received signals will be transmitted over the receiving circuit to the hybrid coil H1 and will be passed through that coil and switchboard SW to the telephone line TL over which they will be transmitted to the west telephone subscriber. A portion of the east-to-west signals will be diverted from the receiving circuit RC at the point 5 passing into the input of the receiving vodas RV. The diverted speech signals, with a level determined by the adjustment of the sensitivity-adjusting pad 6, will be impressed upon the input of amplifier-detector 1 where they will be amplified and detected. The detected signal currents will be supplied as energizing current to the operating winding 42 of the master receiving relay RM and will cause that relay to operate to shift its armature from the left-hand to the right-hand contacts.

As noted above, with ther relay RM in its unoperated condition with its armature held on the left-hand contact, biasing current is sup-plied to the biasing windings of the switching relays S4 and S5 over a circuit extending from ground through plate battery I6, armature and lefthand contact of relay RM, resistance 5I, biasing winding 46 of relay S4, biasing winding 49 of relay S5 and resistance 52 to ground. Under this condition, there is no current flowing in the operating winding 45 of relay S4. and the operating winding 48 of relay S5, as they are connected across the balanced point in the Wheatstone bridge circuit, that is, across the diagonal of the bridge extending from a point between resistances 53 and 54 and a point between resistance 52 and the biasing winding 49 of relay S5.

When the relay RM operates to break its lefthand contact, the connection between battery I6 and the biasing windings of relays Si and S5 is immediately broken so that biasing current ceases to iiow. Then, the bridge being unbalanced, the unbalance current of the bridge will flow through the operating windings of relays S4 and S5, over a series circuit extending from the positive terminal of plate battery I6 through resistance 53, operating winding 45 of relay S4, operating winding 48 of ,relay S5, resistance 52 and ground to the negative terminal of battery I6, causing operation of the relays Si and S5. The effect of simultaneously removing the bias and adding operate current to the relays S4 and S5 results in fast operation of these relays. Relay S5 operates to break its .left-hand contact disabling the vogad VC1 in the transmitting circuit as indicated diagrammatically. Relay S4 operates to remove the battery I6 from the transmitting hang-over circuit, to prevent echoes from falsely operating the transmitting relays.

The operation of relay RM to break its lefthand contact also provides a break in the energizing circuit for the operating windings 25 and 26 of the transmitting master relays TM1 and TMz respectively, from plate battery I6, preventing the subsequent operation of these relays under control of outgoing signals thereafter initiated in the transmitting circuit TC, as long as the retial of battery i6 when relay RM is unoperated,.is.

discharged through the resistance 62.

The disabling off the transmitting vodas relayA chain under control of the receiving. vodas.1R.TiZI in the manner which has just been described', isnot quite positive enoughso that there may,l be a possibility of a receiving volume penalty under certain circuit conditions. When theterminalis set up fory zerov echo margin, its transmitting amplier-detector 4-andthe receiving amplifierdetector 1 have equal sensitivities as-seenirom thel point 2 in the receiving circuit RC. Under these conditions, the detectorsare workingv on speech levels close to their just operating sensitivities, jwhere operating currents are low and variations in relay adjustmentwill cause difference in; relay operate and release times.

In order to overcome this:1 objection, the cir.- cuit arrangement of'Figi l is rranged. to provide a desensitizing action te` the transmittingv vodas under control ofthe receiving vodas inthe .following manner.

As indicated, the anode 2.2 of thedetector tube i3 in the receiving amplifier-detector: 1 is connected through the operating winding 42 of the .master receiving relay RMto'the screen grid I2 of the detector tube 8in the transmitting vodas amplifier-detector Il; When the receiving vodas RV is unoperated, a positive voltage is applied tothis screen grid from plate battery It-through the series resistance 23, so that the-detector tube 8 is in the condition to be operatedquickly in response to applied signals. With the connection of the plate 200i? the receivingvodas detector it to the screen grid i2 of theitransmitting.vodasv detector 3 as described above, when an input signal is applied to the receiving vodas detector i3, the plate current flowing through resistance 23 will cause a voltagev drop whichreduces the voltage on the screen grid l2 of the transmitting vodas detector il a lilreamount, this circuit being proportioned to prevent any plate-current flowin the transmitting master relays 'I'Miand- 'IMz .to more effectively disablethem.

When. the receiving master. rela-y RM releases with cessation in the, supply of received signals to the receiving vodasRV, to restore the relay armature to its leit-handcontact, biasing current is again supplied from battery I6 to the bi ig windings A5' and 49 of relays S4 and- S5, r` ctively, and the bridge being again balanced current through the operating windings of these relays is again reducedtdzero. However, the relays S4 and S5 will be maintained operated fcr a suitable hang-over yinterval due to the holdover action provided by charging current Ilowing to the hold-over condenser 6 Lover a series circuit extending from the positive terminal of battery tfthroughresistance 52, condenser'l, resistance- 60, hold winding. 41 of relay S4, hold winding 50 of relay S5 andground to the negative terminal ofy battery i6.

The quick operation of the transmitting vodas TV and of the receiving vodas RV provided rcspectively by the use of a high normal negative bias on the detector of the transmitting vodas and the fast operating relay circuit in the receiving vodas and by making the operation of the detector in the receiving vodas disable the corresponding detector in the transmitting vodas and the receiving master relay disable the transmitting master relays, provided in the circuit of the invenion shown in Fig. 1, as described above, enables zero echo margin to be maintained at the terminal without resorting to the use of delay circuits in the signal transmission paths. This improvement results in a more satisfactory operation of the terminals in that higher volumes can be delivered to the receiving subscriber.

Fig. 2 shows a modified switching circuit in accordance with the invention associated with a two-Way repeating circuited used for switching losses between the two sides of the repeater and suppressing echoes.

As shown, the repeating circuit comprises a west-to-east transmission circuit EA including the one-way amplifier A1, and an east-to-west transmission circuit WA including the one-way amplifier A2. These circuits may be oppositelydirected one-way repeating circuits of a twowire-repeater or a four-wire telephone circuit, in which case, the circuits EA and WA would be connected at their terminals in conjugate relationl to each other and in energy transmitting relation with a two-wire telephone circuit by a hybrid coil and associated balancing network, or other suitable means (not shown).

Connected across the circuit EA in front of the ampliiier A1 is the input of a switching circuit 65 comprising the one-way amplifying device 66, the detector 61 and a chain of switching relays including the polarized relay ME and the polarized relay HE, controlled by operation of the detector 61. Similarly, connected-across the circuit WA in front of the amplifier A2 is the input of a switching circuit 10 comprising the one-way amplifying device-11, the detector 12and achain of switching rela-ys including the-polarized' relay MW and the polarized-relay HW, controlled by operation-of thedetector 121.

The detector 61. iny switching circuiti 65 comprisesa. single electron discharge tube of the' pentode type having a` cathode 15, which may be: of` the heater type, supplied with'heating' current in. anyy suitable manner, the anode- 15, a control .grid` 1-1, a screen grid 18I and a suppressor gridf19 connected directly to the cathode-15. The control. grid-cathode circuit of tube 61 is coupled to the outputofthe amplifying device 68 byj transformer 80, andthe tube 61 is normally biasedto cut-01T by. the negative potential applied tothe. control grid 11 by agrid battery 8l through the,- secondary. winding 0f transformer 80'.. B'ut in the'presence of `suflicient voice frequency voltage applied through the transformer 8,6; to. the control gridfll, plate current will flow inthe platecircuit. of whichthe valternating current component .is shuntedthrough condenser 82. Plate current. is,supplied.to.theanode 16 of detector tube. (i1. from platebattery. 83. through the biasing winding. 84 of. relay MW and the operatingfwi'nding 85;of. relay.- MEin series.

Similarly., the. detector 12 of the switching circuit 'lilv connected tothe-circuit WA comprises a single electronv discharge device of the pentode typehavingacathode 86, an anode 8.1-, a control grid 'fltalscreen-gridl 8-9 anda suppressor gridv 90 connected directly to the cathode.` The dev tector tube 12 is biased tocut-of! by the negativek potential applied by grid battery 9|'through the secondary winding of the input transformer 92. Plate current is supplied to the anode 81 of detector tube 12 from plate battery 93 through ating winding 85 of relay ME. Similarly, the

screen grid 89 of detector tube 12 in switching circuit 10 is connected directly to the anode 81 so as to be positively biased at the same potential as that anode from plate battery 93 through the biasing winding 94 of relay ME 'and the operating winding 95 of relay MW.

.The amature of the polarized switching relay ME is normally held to itsleft-hand contact by current flowing, from the plate battery 93 through biasing winding 94 and resistance 96 to ground, and the armature of polarized relay MW is normally held on its right-hand contact by the biasing current flowing from the plater battery 83 through biasing winding 84 and resistance 01' to ground. The polarized relay HE has three windings, an operating winding 98, a biasing winding`99 and a hold winding |00. The biasing winding 99 and operating winding 98 of relay HE are connected in a balanced Wheatstone bridge circuit with battery |0|, resistances |02, |03, |04,

' |05 and |06 and the contacts of relay NIE. In this bridge, the closed armature and left-hand contact of' relay ME, biasing winding 99 and resistance |02 are connected in series in one arm, and the resistances |03, |04 and |05 constitute the other three arms. The battery |0| in series with the resistance |05 is connected across one diagonal of the bridge so as to supply the biasing winding f 99 of relay HE with biasing current over a circuit extending from the positive terminal of the battery v'through resistances |06, |03, |02, biasing winding 99, closed lefthand contact and armature of relay ME, and ground to the negative terminal o1 battery I0 The values of the elements of the four arms of the bridge being selected so as to balance the bridge, the operating winding 98 of relay HE whichis connected across the other (balanced) diagonal of the bridge, will normally receive no operating current. The hold winding lili) of relay HE is connected in a circuit which may be traced from ground through battery |01, winding |00, series resistances |08, |09 to the right-hand normallykopen contact of relay ME, a hold-over condenser ||0 being connected from a point between the two resistances |08 and |09 to ground.

Similarly, the polarized relay HW in the switchingcircuit 10 has an operating winding a biasing winding ||2 and a hold winding ||3, the operating and biasing windings being connected with battery |0|, resistances ||4, ||5, |I6, ||1 and H8 and the contacts of relay NW in a balanced Wheatstone bridge circuit. In the latter bridge circuit, one arm comprises the closed armature and right-hand contact of relay MW, the biasing winding ||2 of relay HW and resistance ||4 in series and the other three arms contact of relay MW closed, it supplies biasingl,

current to the biasing winding ||2 of relay HW. With the values of the elements in the four arms of the bridge selected to balance the bridge, the operating winding of relay HW, which is connected across the other balanced diagonal, willi;

receive no operating current. The hold winding |.|3 of relay HW is connected in a circuit which may be traced from ground through battery I9, s eries resistance |20 and |2| to the open lefthand contact of relay MW, a point between theequal series resistances |20 and |2| being conn ected through the hold-over'condenser |22 to ground.

The switching circuits as described above operate as follows. With signal transmission over thel.

When the level of125 rent component of which will pass through the???0 operating winding 85 of relay ME and the biasing winding 84 of relay MW in series in such direction as to oppose the normal bias on relay ME produced by the biasing winding 94. This operating current will cause operation of relay5 ME to shift its armature from the left-hand to the right-hand contact. As soon as the armature of relay ME leaves its left-hand contact the arm of the bridge circuit including the biasing winding 99 of relay HE is opened causing biasing current' tobe removed from that winding and the bridge to be unbalanced. The unbalance current of the bridge will then ow through the operating winding 98 of the relay HE causing operation of the relay to shift its armature from the right-bandi contact to its left-hand contact.

In the idle condition of the repeating circuit of Fig. 2 with no signals being transmitted in either direction, a resistance |23 connected across the transmission circuit EA in the output of the arri--V plifler A1 through the armature and closed righthand contact of relay HE provides a loss of, say, about 5 decibels, in that circuit, and the resistance |24 connected across the transmission circuit WA in the output of amplier A2, through the nor-- mally closed armature and left-hand contact of relay HW provides a loss of equivalent Value in the latter circuit.

The breaking of the right-hand contact of relayL HE in the manner previously described opens the circuit through resistance |23 normally connected across the circuit EA removing the 5 decibels from that circuit and allowing wests speech currents after amplification byamplier A1 to be transmitted with little attenuation out over the circuit EA towards the east subscriber. Because of the fast operation of the switching circuit 65, due to the fast operating relay circuit described, no delay circuit is required with circuit EA in front of the switching point.

When the armature of relay HE reaches its left-hand contact, the resistance |25 is connected across the circuit WA in the input of amplifier A2, and the input of switching circuit 10 is shortcircuited through the left-hand contact and armature of relay HE. The value of resistance 125 f is Amade such that it introduces a desired loss r suliicient to suppress echoes in the circuit WA, but because the resistance of the wiring and the left-hand contact of relay HE does not completely disable switching circuit 'l0 to prevent operation of that circuit by any voice frequency voltages that may be subsequently applied thereto from the path WA. A previous desensitization of switching circuit 'lo occurs when direct current iirst iiows in the anode-cathode circuit of detector tube''l in switching circuit 55 in response to wests speech currents, since the biasing winding 84 of relay MW oi switching circuit 'i0 is in series with the operating winding 85 of relay ME.

When the input level to switching circuit 6-5 falls below a certain value with cessationin the transmission of signals over the path EA, the energizing current applied to the operating winding 85 of relay ME will be reduced to the point where relayME will release to return its armature to its left-hand contact. This restores the bias to relay HEby reconnecting battery IDI to thebiasing winding 99, and balances the bridge circuit 'so that unbalance current through the operating winding 98 of that relay is reduced to Zero. The operate current previously -flowing through hold winding liloY of relay HE, series resistances Hi8 and m9, and the closed armature and right-hand contact of relay AME' to ground is then diverted into condenser Hl) and will continue to flow through 'the hold winding |00 of relay HE and resistance IGS to that condenser until the latter is charged up to the potential of battery l'l, thus holding the .relay HEy operated for Aa .den'ite hangover time interval after the relay ME has released. yAt the end ci that time interval, krelay 'HE will release connecting resistance l23 across the path EA so as to reinsert the normal loss therein, and simultaneously removing resistance 25 from in shunt of the circuit WA and removing the short circuit 'from the input of `the switchingcircuit 'I0 so that the switching circuits arereturnedtcvthe normal condition shown in Fig. 2.

As the apparatus in switching circuits 65 and 'E0 are identicahthe'manner of operation for the case when the east subscribers speech current arrives atthe input of switching circuit l over the circuit WA before speech currents are received by the switching circuit 65 over the transmission circuit EAfrom the west subscriber, will be similar yto-that previously described for the opposite case, and therefore need not be described here.

`Various modifications of the circuits illustrated and vdescribed which are within Vthe spirit and scope of the invention will be apparent to persons skilled in the art. It should be understood that 'the specied values for the losses inserted in the transmission path are given by way of example only, and that they may be quitediferent from thevalues givendepending on the characteristics Iof the signal transmission circuits with which the switching circuits are used.

-Whatis claimed is:

1. A relay-circuit comprising a relay having a biasing windingand an operating winding, abalanced bridge including said biasing winding in one of its arms,a source of current connected across one diagonal of'said bridge and normally supply- `ingicurrent to saidbiasing winding in such direcsponse to the flow of the unbalance current of said bridge through said operating winding.

2. A'relay circuit comprising a relay having a biasing winding, an operating winding and a holding winding, a lbalanced bridge including said biasing winding in one of its arms, a source of current connected across one diagonal of said bridge, supplying current to said biasing winding in such direction as to bias said relay against operation, said operating winding being connected across the 'other diagonal of said bridge, and means to open said arm of said bridge including said biasing winding to disconnect said source from said biasing winding, to cause fast operation of said relay by the resulting unbalance current of said bridge through said operating winding, andthen to close an energizing circuit for said holding winding to hold, said relay in the operated condition.

3. 'In a signal-operated switching circuit, a relay having an operating winding, a biasing winding, a kbalanced bridge including said biasing winding in one oi its four arms, a source of current connected across one diagonal of said bridge supplying current to said biasing winding in such direction as 4to bias said relay against operation, said operating winding of said relay being connected across the other diagonal of said bridge,

and means responsive to received signals to open said one arm of said bridge to disconnect said source of current from'said biasing winding and to unbalancefsaid'bridge so that the flow of unbalance current through said operating winding causes'quick operation of said relay.

Ll. A signal-operated switching circuit compris- 'inga control relay having an armature and two contacts, a switching relay having a biasing Winding, 'an operating winding and a holding Winding, .a `plurality of resistances, said biasing winding and said resistances in the unoperated condition of `said control relay Yforming 'a balanced bridge circuit through the closed armature and one contact of said control relay in one arm, a source of current connected across one diagonal of said bridge normally supplying current to said biasing winding in such direction as to bias said switchingrelay against'operation, said operating -windingbeing connected across the other diagonal of said bridge, and signal-controlled means for operating said control relay from said one to its other contact, the ybreak of said one contact disconnecting said source of current from said biasing winding and unbalancing said bridge to 

